I would think so....my friend got pregnant within 3 wks of having her baby and Dr asked her if she planned on nursing 2 when she was in the end of her pregnancy....he never said it was a bad idea. She did nurse both for about 6 months and they are both fine. i would think he woulda discouraged it if it was not ok.

I think your body will adapt to individual needs. Isn't that the whole concept of breastmilk? Granted, I don't know HOW it would figure that out since you won't be the pregnant one, but like many others have said, any BM is better than none. Your body would naturally pick up supply (I'd think) when you start tandem nursing.
FWIW, I nursed my DD through the end of my pregnancy, DS got colostrum at birth, and then it switched back to mature milk. Our bodies are amazing things!

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Nicole, mom behind Napping House Yarns, wife to C - mother to Grace and IsaiahExpecting a sweet little BOY on September 17th, 2012
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I think that once the newborn starts nursing, the frequency of the nursing will increase the milk supply, and that will tell the mother's body to make milk for the newborn with the appropriate fat/protein content. The mother may have to supplement until her supply catches up with demand, but if she knows in advance when the baby will come, she can probably pump to increase supply in advance.